Digital printing presses and other digitally fed printing machines are widely used and are made in a great variety of types and models. They vary in terms of mechanical configuration, the basic process utilized for marking, the types and formats of media being printed and the nature of the printed images. These variables are inter-related. The present invention is applicable to printing machines of almost any type, all of which will be referred to hereinafter interchangeably as digital printers or just printers, and constitutes an improvement thereto, which may be advantageous for certain applications, as explained hereunder.
Common to all such printers is the presence of a medium to be imprinted and of a printhead. The media to be imprinted may consist of any of a variety of materials, including paper, cardboard, plastics, metal, textiles, ceramics, etc., and may have any of a variety of formats and sizes, including cut or rolled-up sheets, plates, tiles and formed products or parts thereof. A printhead includes a printing device, or an assembly of printing devices, that faces the medium and, under control of suitable signals, causes image-related marks to be left thereon. This process is referred to as marking or printing. The printhead is primarily classified by the basic type of the marking process and by the mode in which the marking proceeds. Marking generally involves some relative motion between the printhead and the medium in a plane parallel to the printed face of the medium. Generally this motion is along two orthogonal axes, usually being relatively fast along one axis, say X axis, (this motion also referred to as a sweep motion) and relatively slow along the other axis, say Y axis, (this motion being either continuous or stepwise), such a combined motion tracing a rectangular raster of lines. In the following description these motions will sometimes be referred to simply as “fast” and “slow” motions, respectively. However, for certain types of printheads and modes of marking it need be along only one axis, while for certain other types or modes it may be at similar rates along both axes (the trace not forming a raster). There will now be described examples of commonly used general types of printheads and their related marking processes and tracing modes.
The presently most ubiquitous marking process is known as the ink-jet process, which may be of two basic types—the so-called continuous ink jet (CIJ) process and the so-called drop-on-demand (DOD) process. An ink-jet printhead may include one or more ink-jet devices, each device emitting drops from one or more nozzles or apertures; in the case of a plurality of nozzles or apertures (which is prevalent for the DOD type), they usually form a regular array. Often, a plurality of ink-jet devices is assembled into a single printhead, forming a regular array, and if each device has an array of apertures, the assembly is such that all the arrays effectively combine into one large array of apertures. The effect of the array is that during the fast relative motion between the printhead and the medium along one axis, the marking by the several apertures is along corresponding parallel traces, which are usually equispaced and span the width of the printhead array. Generally, this width is much less than that of the image to be printed, so that a slow relative motion between the printhead and the medium is required also along the other axis to cover the whole width of the image. Also generally the spacing of the traces is coarser than the desired printing resolution; the slow motion along the other axis is then such that traces of consecutive sweeps become mutually interlaced. In certain types of digital presses (such as the Idanit digital press by Scitex Vision), the printhead is made to span the maximum width of the media and thus the slow motion serves only for interlacing of traces. Another type of marking device that requires two-axes motion, possibly in a non-raster mode, is an air brush. It is used for special low-resolution printing (or image-painting) applications.
A group of printing device types based on optical processes is also known. In these processes, marking is generally achieved in two stages: during a first (exposure) phase, one or more focused light beams, emerging from the printhead modulated by control signals, strike the medium or an intermediate surface, leaving thereon a latent image. During a second (development) stage, the latent image becomes a visible image on the medium. Two main types of exposure devices, and thus of optical printheads, are prevalent: the first main type consists of an array of modulated light sources, such as light-emitting diodes (LEDs); its mode of tracing is similar to that of an ink-jet array, generally requiring raster-like motion along both axes. The second main type has an intense beam of light, usually emanating from a laser, that is modulated and swept across the image area; here mechanical slow motion is required only along one axis. It is noted that the term light is used here to denote any focusable electromagnetic radiation and thus includes also ultra-violet and infra-red radiation. It is further noted that the marking process need not be based on photoelectric or photoconductive effects, but may for example be based on thermal effects.
Array-like printing devices using physical processes other than those discussed above are also known, such as those using direct thermal effects or direct electrostatic charging effects. Swept-beam printing devices using other than light beams, such as electron- or ion beams, are likewise known. Digital printers based on such and other devices are likewise subject to the improvements disclosed herein.
The marks left by the printing process on the medium may be any optically readable marks, such as those made by ink, paint or toner, or they may be any other material or effect on the medium, such as a varnish, a masking industrial layer or an etching, and the like. In the case of optically readable marks, the several devices in a printhead may include devices that mark in different colors. This is especially true for ink-jet (as well as air-brush) printing, where the inks themselves are colored. Such inks may be in the four primary printing colors or have any other desirable colors and constituent materials, including metallic and fluorescent materials. Digital printers based on such and other printing processes are likewise subject to the improvements disclosed herein.
Printers are mechanically differentiated by the manner in which the relative motion of the printhead and medium are carried out. There are three basic mechanical arrangements related to such motion. In a first arrangement, the medium is stationary during the printing of an image and the printhead is generally movable along the two orthogonal axes—usually in a relatively fast motion along the X axis and in a relatively slow motion along the Y axis. Often the medium is a sheet or a plate that lies flat, in which case this arrangement is also termed flat-bed printer. In the case of a swept-beam type of printhead, the sweep assumed to be along the X axis, there is only a slow mechanical motion along the Y axis. In the case of an array-type printhead that spans the entire maximal width of a printed image, the motion along the Y axis need only be for trace interlacing, as explained above. Any motion of a printhead during marking will be referred to as a marking motion.
In a second mechanical arrangement, the medium moves slowly along the Y axis, while the printhead generally moves repeatedly along the X axis, in a relatively fast motion. In the case of a swept-beam type of printhead, the printhead is stationary, the sweep being aligned with the X axis. Digital printers of this second basic arrangement vary according to whether the printed medium is flexible or rigid, and if flexible—whether it is in the form of a plurality of separate sheets or formed into a very long sheet, also known as a web. The case of a rigid medium also includes flexible media, such as one or more garments, that are attached to, or mounted on, a rigid substrate. A rigid medium or substrate is usually flat and during printing moves parallel to one of its coordinates; this may be regarded as another configuration of a flat-bed printer. A rigid medium or substrate may, however, also have another convenient shape, such as a cylinder; in the latter case it slowly rotates around its axis, while the printhead moves fast parallel to the axis of rotation. A web-formed medium moves from reel to reel, past a printing station, by means of rollers; at the printing station it is stretched to become planar or is made to run in contact with a backing surface. A flexible sheet is moved past a printing station either by means of rollers or temporarily attached to a substrate, which may be flexible (such as an endless belt) or rigid (such as a cylinder).
In a third mechanical arrangement, it is the medium that moves fast, e.g. attached to a rotating cylinder, while the printhead generally moves in a relatively slow motion. If the printhead includes an array that spans the width of the printed image, the slow motion need only be for trace interlacing, as explained above. It will be appreciated that a fourth basic mechanical arrangement is theoretically possible, though generally not practical nor known to be practiced, namely a stationary printhead with a medium moving along both orthogonal axes; the invention is applicable to such an arrangement, as well as to all the others mentioned hereabove, with obvious modifications, which would, moreover, be relatively simple to embody.
For each of the above arrangements there are known a variety of ways for loading the medium (i.e. bringing the medium into the general area of printing), moving it during marking and unloading it (i.e. taking the medium out of that area). In the cases of a rigid medium, or substrate, and a sheet-formed flexible medium, the motions required for loading and unloading are distinct from, and generally faster than, the aforementioned slow motion during marking. In the case of a web-formed medium all three motions have the same average rate but may be separately controlled; this is particularly apparent if the motion for marking is stepwise. There also is a possibility that the printer is but one station in a production line, where other stations may include similar printers or may involve other processes. In a configuration involving a web, the web may then continuously run into the printer from a preceding workstation and out of the printer into the next workstation. In configurations involving sheets or plates (including the case of substrates that carry pieces to be printed), the latter may be moved from one station to another, for example, in a round-robin fashion, whereby one or two stations may serve to load and unload the pieces or the substrates. It is noted that flat-bed configurations are useful for printing a large variety of media, particularly rigid ones or such that consist of fabricated pieces attached to a substrate. For any of the above ways of moving the media, the present invention is applicable with respect to the motion of the media during the marking process.
There are applications in which it is required to print, or image-wise paint, curved surfaces. These may, for example, be outside surfaces of various objects that cannot be fabricated by cutting, folding and gluing a flat medium (e.g. cardboard). To this end, a printer of any of the arrangements discussed above may be modified to allow relative motion between the printhead and the medium also along a third orthogonal axis, say—the Z axis. The motion along the Z axis is then controlled so that the distance between the printhead and the area of the medium being imprinted remains constant.
Essentially all printers of prior art are equipped, and designed to function, with a single printhead. The term printhead in this context is to be understood as any printhead of the types described hereabove, and similar ones, characterized by being mechanically a single assembly and operative to mark essentially the entire printable area of the medium, while the latter is in the printing position. Typically, the printhead gradually marks an entire image, as the aforementioned relative motion between it and the medium takes place. If the printhead includes an array of marking devices, they are arranged so as to mark parallel traces that are relatively close to each other and, as noted above, successive sweeps generally cause these traces to interlace. In the case of multiple color devices in a single printhead, they are generally arranged so that their traces overlap each other on successive sweeps.
There are many applications in which a plurality of separate images, often identical ones, need to be printed on a single medium. The multiplicity may be along the X axis, along the Y axis or along both. This need arises particularly where an array of discrete pieces of print media must be printed. Typical examples are decorative tiles, T-shirts, peel-and-stick labels. Yet other examples are multiple copies of a poster or leaflet, as well as of pages of a book, to be printed on a single sheet.
Clearly, all such printing jobs can be carried out in conventional single-printhead printers, by suitably programming the control signals. Such an operation may have two drawbacks: first, in many cases there are relatively large spaces between the printed pieces or between the page images, in which no marking is to take place; the time during which the printhead sweeps over these spaces is wasted—resulting in reduced utility of the printer. While speeding up the motion of the printhead or of the medium over these spaces is theoretically possible, it may not be practical, because of the high rates of acceleration and deceleration required. Secondly, since the multiple images are marked sequentially, the time it takes to mark all of them is that multiple of the time that it takes to mark any one of them, so that marking them sequentially using a single printhead is disadvantageous relative to marking several images simultaneously using multiple printheads.
The overall printing rate of a given printer may generally be increased by increasing the sweeping speed during marking or by increasing the number of printing devices operating simultaneously. The sweeping speed is ultimately limited by mechanical considerations and by the maximal marking rate of each device. Increasing the number of marking devices in a printhead would result in an increased number of traces marked per sweep. This would require, with respect to the Y axis, a commensurate increase in speed, in the case of continuous motion, or a commensurate increase in the step size; in either case, the mechanical precision required to maintain alignment between successive sweeps may be taxed. If the number of marking devices in the printhead is increased to span the whole width of the medium (thus requiring very little motion, if any, along the Y axis, as is the case in certain printers of the third basic arrangement, as explained above), there may be a considerable number of devices (or portions of such devices) that trace only spaces between images and therefore represent a wasteful investment.
In the case of curved surfaces to be printed, which requires also motion along the Z axis, there is a limitation on the size and number of printing devices in any one printhead: it must be small enough for the distance that is maintained between the printhead and the curved surface to be practically the same for all the devices and apertures.
It is further noted that in multiple-image applications, the size of the images, as well as the width of the gaps between them, may be variable—both between jobs and between images on the same sheet. Overcoming the investment inefficiency of a full-width array printhead, as suggested hereabove, by leaving out some of the marking devices, would be impractical in view of this variability.
It is furthermore noted that in some multiple-image applications, the various images may have to be printed on different media; for example, a batch of T-shirts to be imprinted may include samples made of different materials, or as another example, a fabricated object may include parts made of different materials. Such different media would need suitably different types of printing devices or inks and thus could not be printed by a single printhead in a single operation. Using a conventional printer, the job will have to be done in several runs—possibly on different printers. Alternatively, the printhead of a single printer could be equipped with several different printing devices (or devices with several different inks) and the job done over that number of printing operations. Obviously such operation would be very wasteful of the printer's time.
There is thus a clear need for digital printer configurations that would enable printing multiple images, of various sizes, at higher efficiency and considerably higher effective rates than possible with corresponding configurations of prior art.